Inductive measurement of physical parameters in a metallic and electrically conductive material is previously known. With an inductive measurement technique, both the distance to and the dimension of a metallic object may be determined. The method also provides a possibility of measuring the electrical resistivity of the material and determining parameters such as the thickness of sheet and pipe walls. U.S. Pat. No. 4475083 for example, it is described how a magnetic field, created by sinusoidal time variation, may be used for measuring distance, dimension and electrical resistivity. By studying the decay of a magnetic field created by means of a square pulse, U.S. Pat. No. 5059902 describes how such technique may be used for determining distance, dimension, electrical resistivity and material thick-ness. From U.S. Pat. No. 5270646 it is also known how the width and the edge position of a rolled billet can be determined by analyzing, having a measurement device with a plurality of measuring coils, the decay of a magnetic field created around the blank to be rolled.
The evaluation technique which is used can briefly be described as follows. A magnetizing field is generated with the aid of a primary coil which can be fed with alternating current or a pulsed bipolar or unidirectional direct current. By evaluating the aperiodic voltage signal which is induced in a measuring coil and which arises in connection with the decaying magnetic field, the distance between the measuring coil and the electrically conductive material can be determined as well as the thickness and the electrical conductivity of the material. The parameters can be substantially determined by sampled measurement, that is, dividing the measurement into different time gaps. The distance between the coil and the material is substantially determined by the quantity of the voltage induced in a time gap immediately after the magnetic field has been shut off.
In most applications, the known methods of measurement function satisfactorily. However, problems arise when the object to be measured is disposed between two contact surfaces to form a closed electric circuit. When the current in the primary coil is broken, a decaying magnetic field arises, which induces eddy currents in the object to be measured. Those eddy currents influence the decaying magnetic field in a way which is characteristic of the object to be measured. The decay process is sensed with the measuring coil, in which the magnetic field induces a voltage, from which the sought parameters can be analyzed. However, the same magnetic field also induces a current in the above-mentioned circuit. The current in this electric circuit in turn causes a magnetic field which counteracts the decaying magnetic field and thus supplies an error in the induced voltage in the measuring coil. By analyzing only the voltage in the measuring coil, the information-carrying signal cannot be distinguished from the interference signal. This can lead to considerable errors in the parameters to be determined. The problem is accentuated when the contact surfaces are located near the measuring coil so that the external closed electric circuit tightly surrounds the measurement range.
During machining of a blank to be rolled, the blank is in continuous contact with the rolls, which in turn are connected to each other via a supporting system of beams or ground. In this way, an external closed electric circuit arises, whereby the blank to be rolled constitutes one of the conductors in the circuit. When applying the known method of measurement, considerable errors may occur when determining, for example, the thickness of the blank to be rolled.